The invention relates generally to the field of multiple input antenna systems and more particularly to the simultaneous use of a single antenna array by pluralities of independent and isolated radio devices.
There have been a number of different solutions to the basic problem of simultaneously using a single antenna structure in conjunction with a number of independent transmitters while maintaining isolation therebetween.
One prior solution uses a single omnidirectional antenna and couples each of the transmitters to this antenna through an associated resonant cavity. Thus an omnidirectional radiation pattern is obtained for each transmitter and the output of each transmitter will not affect the output of any other transmitter. The primary disadvantage of this system is that it requires a separate tuned resonant cavity for each transmitter. Since every transmitter must operate at a substantially different frequency in order for the resonant cavities to provide the required isolation, close channel spacing in such a system is impractical. Also, because of the requirement for a tuned resonant cavity, such a system has an inherently narrow bandwidth. In addition, the resonant cavity must be adjusted whenever the operating center frequency of a transmitter is changed.
Another solution to the problem uses an array of hybrid networks to produce a single output signal which is then used to excite a single omnidirectional antenna element. Each of these hybrid networks combines two input signals to produce a half power output signal at one port while dissipating the rest of the power in a 50 ohm "dummy" load. In a typical eight transmitter network built according to this prior technique, a 9db loss in power is encountered. The use of hybrid networks does, however, provide isolation between the independent transmitters as well as permitting a wide bandwidth of operation for the antenna system.
Still another solution to the problem is to couple individual omnidirectional antenna elements to each of the transmitters. This solution is not practical because of the large separation that would have to exist between each of the radiating elements in order to provide sufficient isolation between each of the transmitters. Therefore the resultant antenna system would require an extremely large amount of space, especially if a large number of independent transmitters were desired.
In still another solution to the problem, two transmitters are combined by a single hybrid network and the two output signals from this network are then used to excite the two independent antenna elements in a turnstile antenna. This technique provides isolation in addition to a wide bandwidth of operation, but cannot be readily extended to more than two transmitters without using narrow band tuned elements or sacrificing a substantial amount of transmitter output power.
In the copending U.S. Pat. application Ser. No. 601,560, referred to above, a particular solution is described which provides a vast improvement over the previously mentioned prior art systems. This copending U.S. application, which is hereby incorporated by reference into the specification of the present invention, discloses the use of a hybrid combining network for receiving a plurality of independent signal sources and producing a plurality of output signals which are each coupled to an associate independent antenna element. These antenna elements each create an independent radiation pattern and these radiation patterns combine to form single composite radiation patterns for each of the independent signal sources. However, the effectiveness of this system is decreased as additional signal sources are added to the system, since these signal sources will result in an antenna array having a very large overall dimension. In addition, the complexity of the combining network is greatly increased when more than 4 independent signal sources are to be combined.